Electric vehicles utilize batteries or other electrical storage devices to store electrical energy that is used to provide power to the electric vehicles' drive train. When the stored energy in the battery is depleted, the battery must be recharged. An electric vehicle supply equipment (EVSE) is utilized to connect electric vehicles to an electrical distribution system for the purpose of charging the battery. An EVSE may also be referred to as a supply equipment, a vehicle charger, a charging station, or a charger.
One potential problem with electric vehicles is that high voltages and currents are generally required to charge the batteries in the electric vehicles. Such high voltages and currents present serious safety concerns. For example, a ground fault can occur when an operator of the EVSE inadvertently provides an unintended conduction path to ground, creating an electrical shock hazard. There are a number of potential causes for a ground fault such as, for example, the insulation on a wire becoming damaged or a component of the EVSE coming into contact with water or other liquids. Additionally, the metallic surfaces and components of the electric vehicle are potential sources for an electrical shock hazard. Indeed, because the electric vehicle is insulated from the ground due to the vehicle's rubber tires, the electric vehicle itself can present a hazard if not properly grounded during recharging of the battery.
To address such safety concerns, a number of standards have been published specifying safety features for an EVSE. For example, the UL 2231 standard requires that the EVSE include a charging circuit interrupting device (CCID) to disconnect the source of power if a potential ground fault condition is detected. In one implementation, the EVSE includes a single CCID configured to have a rated trip threshold of 5 mA (i.e., a “CCID5”) to protect the load-side of the charging system at a level below a maximum “let-go” current level. A maximum “let-go” current level is the current level at which the electricity causes a person's muscles to contract to the point that the person loses control of their muscles and, thus, cannot let go of a conductor until the current is removed. Accordingly, when a system is protected below the maximum let-go current level (i.e., at let-go current levels), the risk of serious bodily harm is substantially reduced. While the maximum let-go current level differs for each person, an average maximum let go level is generally in a range from approximately 9 mA to approximately 15 mA.
Unfortunately, conventional systems employing a CCID5 suffer from a significant drawback. For various reasons (e.g., degraded insulation, accumulated dirt or salt, corrosion, natural leakage through EMI related components, etc.), current can leak from the electrical components within the electric vehicle at leakage current levels above the 5 mA trip threshold of the CCID5. Although these leakage currents may be safe (due to grounding of the vehicle chassis), they can cause inconvenient and unacceptable nuisance tripping that interrupts the recharging of the electric vehicle. As a result, the UL 2231 standard alternatively provides for protection utilizing a single CCID20, which has a rated trip threshold level of 20 mA (i.e., a trip threshold level that is generally higher than most nonhazardous leakage currents). Because the rated trip threshold level of the CCID20 is above the maximum let-go current level, the UL 2231 standard requires that the system where the grounding integrity may be compromised also include a ground monitor that is configured to cause the system to interrupt the power if an inadequate connection of the grounding system is detected. That is, the verification of a connection of the electric vehicle and EVSE to ground protects a person from a ground fault occurring internally within the electric vehicle.
Significantly, however, the conventional CCID20 system does not protect a person from a ground fault occurring externally to the electric vehicle at let-go current levels. Rather, the CCID20 system detects a ground fault external to the electrical vehicle only after the ground fault exceeds the 20 mA trip threshold of the CCID20, which is generally above the maximum let-go current level. Thus, with conventional EVSE protection systems, one must choose between a system that experiences excessively inconvenient nuisance tripping and a system that does not protect an operator from ground faults external to the electric vehicle at let-go current levels.